The overall goal of this proposal is to evaluate the safety and therapeutic potential of in vivo blockade of interactions between the inhibitory receptor, PD-1 (Programmed death-1), and its ligands using a SIV/macaque model. Our hypothesis is that blockade of interactions between PD-1 and its ligands can provide therapeutic benefit by eliciting cell-mediated immunity capable of providing better viral control than the functionally limited T cells characteristic of chronic SIV infections. Our recent studies demonstrated that in vivo blockade of PD-1 during chronic SIV infection is safe and restores function of SIV-specific cellular and humoral immunity (velu et. al., Nature 2009). Importantly, this functional restoration of anti-viral immunity was associated with enhanced control of SIV infection. These results clearly demonstrated that in vivo blockade of PD-1 represents a novel treatment strategy to control HIV infection in humans. In this competing renewal we aim to further improve the therapeutic potential of in vivo PD-1 blockade to treat chronic HIV infection using the SIV/rhesus macaque model. We propose to use three approaches. First, we would like to combine the PD-1 blockade with anti-retroviral therapy (ART). This will allow us to perform shorter periods of in vivo blockade and will provide translational potential into HIV-infected humans. Second, we would like to combine the PD-1 blockade (blocking a negative signal) with an approach that stimulates a positive signal and works synergistically for enhancing anti-SIV immunity and viral control. For this purpose we will use the cytokine IL-2 that positively regulates proliferation, survival and perforin expression of virus-specific CD8 T cells. We will test the hypothesis that combining IL-2 therapy with in vivo PD-1 blockade and ART will further enhance the function of virus-specific cellular immunity and control of SIV infection. Third, we propose to characterize the role of other inhibitory receptors in regulating T cell exhaustion during chronic SIV infection and study the synergy between these and PD-1 in vitro, and develop novel treatment strategies that target PD-1 and other inhibitory receptor/s in vivo for further enhancing SIV-specific cellular immunity and control of chronic SIV infection (specific aim 2). We will test the hypothesis that combined in vivo blockade of PD-1 and other inhibitory receptors during chronic SIV infection will further enhance the function of virus-specific cellular immunity and control of SIV infection. Using in vitro blockade experiments, we will define an optimal combination of inhibitory receptor/s that will synergize with PD-1 blockade for enhancing SIV-specific cellular immunity. We will then use the best combination from in vitro studies and perform in vivo blockade during chronic SIV infection for enhancing SIV-specific immunity and viral control. By completion of these aims we hope to have further enhanced the therapeutic potential of in vivo PD-1 blockade to control chronic SIV infection.